Switch mode converters are useful for many applications, including for example ones found in computers operating from 47 Hz to 63 Hz AC mains input voltage, flight avionics systems operating from 360 Hz to 440 Hz on board AC generators, network and telecom line cards operating from positive or negative battery plant DC input voltages, and portable electronic devices such as cellular phones and laptop computers, both of which are primarily supplied with power from batteries. Such electronic devices often contain several sub-circuits, each with its own output voltage level requirement different from that supplied by the input source voltage or battery (sometimes at a higher or lower level than the input source voltage, and possibly even at a negative voltage). Additionally, the input source voltage can surge or sag when connected to a load or the battery voltage increases while being charged, or can decline as its stored energy is delivered to the load. Switch mode DC to DC converters offer a method of stabilizing the output voltage derived from a partially raised or lowered or otherwise slowly varying input source voltage.
Switch mode converters typically convert one DC voltage level to another by storing the energy from the input source voltage temporarily and then releasing that energy to the output at a different voltage. For all but the lowest power converters, energy is stored into and then released from a magnetic components such as an inductor or transformer. Switch mode conversion is more power efficient (often 75% to 98%) than linear voltage regulation (which dissipates unwanted power as heat). This efficiency is beneficial by reducing loading on the electrical grid or increasing the running time of battery operated devices. During each cycle, when the amount of energy temporarily stored equals the amount of energy released, the converter is considered to be in equilibrium.
In these switch mode converters, energy is periodically stored into and released from a magnetic field in an inductor or transformer, typically in but not limited to the range from about 20 kHz to 20 MHz. By adjusting the duty cycle of the switching from the input source voltage, that is the ratio of on/off time, the amount of power transferred can be controlled. Usually, this is done to control the output voltage, though it could be done to control the input current, the output current, or maintaining a constant power. Transformer based converters may provide isolation between the input and the output, and are also typically used when the output is significantly greater than or significantly less than the input source voltage, or the output voltage polarity is inverted relative to the input source voltage polarity.
In general, the terms “DC to DC converter” and “switch mode converter” each refers to one of these switching converters. Many topologies exist including but not limited to non-isolated buck, boost, buck-boost, SEPIC, and Cuk converters or isolated flyback, forward, push pull, and bridge converters.
Magnetic switch mode converters are usually operated in either one of two modes, according to the current in its magnetic energy storage components (inductors or transformer):
continuous in which the current fluctuates but never goes down to zero, and
discontinuous when the current fluctuates during the cycle, going down to zero at or before the end of each cycle
Such converters are usually designed to operate in continuous mode at high power, and in discontinuous mode at low power. For system level electrical noise and acoustic noise and other considerations familiar to those skilled in the art, a converter can also be controlled and operated in continuous mode at low power. When the magnetic switch mode converter is operating in equilibrium the magnetic energy storage device which can be either an inductor or a transformer is considered to also be “volt second balanced.” Should the amount of energy temporarily stored not equal the amount of energy released for each cycle, an imbalance is considered to have occurred. This imbalance can be exacerbated in higher input voltage applications. One approach for balancing high voltage hysteretic buck regulators in higher input voltage applications is described in “Application Challenges of High Voltage Hysteretic Buck Regulators” by Bob Bell of National Semiconductor of Santa Clara, Calif. (publication date and source unknown). Another example of a buck converter is shown in U.S. Pat. No. 6,747,441.